Light straight run naphtha, virgin naphtha and other refinery streams containing C.sub.5 and C.sub.6 paraffins, are useful sources of blending stock for gasoline pools, but, under current market conditions and ecological considerations, it is necessary to raise the Research Octane Number (RON) of the stock to about 85 to 94 (RON), without the addition of alkyl lead compounds. Since the phase-out of lead compounds began, refiners have relied on isomerization and reforming, e.g., platforming, steps to increase the octane to above about 80 RON and further, have integrated separation processes to separate the isomerate or reformate into higher and lower octane fractions, with the higher octane fraction typically about 90 RON, being sent to the gasoline pool. The lower octane fraction(s) may be recycled to the isomerization step for further conversion and higher yields of multi-branched, high octane paraffins.
In one approach to the upgrading of isomerate streams, U.S. Pat. Nos. 4,717,784 and 4,855,529 disclose similar processes for isomerizing a hydrocarbon feed and separating the dimethyl branched paraffins produced in the reaction from the n-paraffins and monomethyl branched paraffins produced by utilizing a size- or shape-selective adsorbent. In the adsorptive separation step, the straight chain and singly branched paraffins are adsorbed into the pores of the molecular sieve while the more highly branched paraffins are excluded from entering the pores. The selectively adsorbed straight chain paraffins and/or the singly branched paraffins are recycled to the isomerization zone for further conversion to more highly branched paraffins. In this separation, the useful adsorbents have pore sizes in the range from 4.5.times.4.5 .ANG. to 5.5.times.5.5 .ANG. which exclude the di-branched paraffins from the pores of the molecular sieve. The preferred desorbent in the processes is hydrogen. The patentees teach that only sieves within this range are operative and others having pore sizes outside the range are too small or too large to perform the separation.
Neuzil U.S. Pat. No. 3,706,813 discloses the separation of dimethyl butanes from methyl pentanes and normal hexanes by selectively adsorbing the multi-branched components on barium-exchanged X or Y zeolites containing from 5-8% (wt.) water. However, adsorbents requiring these high levels of water could not be used for the instant process, because the isomerization catalysts are quickly deactivated by the presence of moisture. In the process of Neuzil, the multi-branched paraffin extract product would be contaminated with water removed from the adsorbent and, hence, could not be recycled to the isomerization reaction zone without first drying the extract stream. The process of our invention avoids this problem since the adsorbents of the invention are capable of selectively adsorbing the dialkyl paraffins without requiring the presence of water. Furthermore, the preferred iso-paraffin desorbents were not effective as desorbents in applicants' process, since no separation could be observed when iso-paraffins were used.
Van Nordstrand et al, Perspectives in Molecular Sieve Science, Flank and Whyte, Ed., Chap. 15 (1988) pp 236-245 found that SAPO-5, AIPO.sub.4 -5 and SSZ-24 have a preference for adsorbing 2,2-dimethyl butane over n-hexane. The data obtained by the "Pore Probe" technique of Santilli (Reference No. 20 of Van Nordstrand et al), indicate intermediate selectivity for 3-methyl pentane. Nothing is contained in the disclosure regarding desorbents necessary for a continuous adsorptive separation process.
Isomerization processes for converting normal paraffins to mono-and di- or more highly branched chain paraffins are commonly used as a method for increasing the octane rating of refinery streams containing normal paraffins. Illustrative of the isomerization process are U.S. Pat. Nos. 3,755,144 to Asselin and 2,966,528 to Haensel.
U.S. Pat. No. 3,755,144 also discloses the separation of normal paraffins from the isomerization reactor effluent using a molecular sieve in a simulated moving bed system and recycling the normal paraffins to the isomerization reactor after recovering the desorbent. Iso-hexane is recovered from the extract in a deisohexanizer column and recycled to the isomerization reactor; iso-pentane and dimethyl butane are taken overhead while cyclic paraffins are recovered in the bottoms of the deisohexanizer. U.S. Pat. No. 2,966,528 further discloses a swing bed system operating under liquid phase conditions for the separation of normal hydrocarbons from branched hydrocarbons in which the normal hydrocarbons are selectively absorbed and desorbed from the adsorbent with a normal paraffin desorbent.
Accordingly, it is an object of the invention to provide a process for continuously separating di-branched paraffins from normal paraffins and mono-branched paraffins produced by an isomerization reaction with a particular adsorbent/desorbent combination by which the di-branched paraffins having a higher octane number are selectively adsorbed onto the adsorbent and low octane components, mono-branched paraffins and normal paraffins, are relatively selectively non-adsorbed by the adsorbent and are recovered from the non-selective void volume as raffinate. The low octane raffinate components are recycled to the isomerization step for further upgrading. The high octane di-branched paraffins are desorbed from the selective pore volume by a carefully selected desorbent. It is a further object of this invention to provide a novel process for combining and integrating an isomerization reaction with a continuous adsorptive separation process for enhanced octane rating of the product stream.